When the amp "stops working" everything but the amplifier appears to work: all the LEDs turn on, external devices plugged into the switched and non-switched 120 VAC ports energize, and the signal path out of the "tape 1 out" works.

There's no visible dust inside the amp. I don't think it's a loose wire because "pretend" vacuuming (or shaking for that matter) has no effect.

Hi ,BDOGs here. The hk620 is an excellent amp ,I have had the same issue . This unit has a hyper sensitive protection circuit . The cause of the issue is buildup of oxidation on wire ends and connectors including RCA wire connectors. Replace your speaker wires or trim them down to fresh looking copper, get some RadioShack tv tuner cleaner spray(they still make it ) and apply sparingly to the small hole of all potentiometers(volume tone balance controls) and turn them from 0 to 10 repeatedly. Also the speaker select buttons and speaker connector posts. As well , clean the RCA input connectors using the spray and a cloth . If you're using old RCA cables replace them , even a poor connection at this point will cause the protection circuit to activate. I hope you haven't given up on this amp, I nearly sledgehammered mine as I couldn't figure out what the hell was causing the shutdown. In a moment of reason I figured going to town on every connection point was the only thing keeping it from becoming a doorstop or landfill . I purchased my hk620 in 1996 and have never found anything comparable for less than 1000 dollars that could come close . I am glad I resurrected mine and this might help,you do the same . Happy listening !

It's probably not the vacuuming, but the disconnecting from AC power for a period of time that's fixing the amp temporarily. Without further troubleshooting it's hard to say for sure, but there might be a failing electrolytic capacitor: look for a swollen or bulging capacitor.

If not a capacitor: it sounds thermally related to me. Next time it fails have some freon spray at the ready to see if spraying at targeted areas inside causes it to spring to life again.

You also can't count out a dirty volume potentiometer: are you hearing any crackling or popping when you change volume now? As @uncle frogy writes above: squirt it with contact cleaner. Or just crank the volume knob back and forth lots.

Bring it to the next Fixit Clnic and we'll take a look: http://www.facebook.com/FixitClinic

Like was said before I do not have enough info to be sure what is going on but I can share my experience with fixing amps that lose a channel or stop working (go silent). the most common problem especially in amps that had crackles or were "scratchy" is dirt or grit in the pots and or switches. fix is cleaning them out with a good safe spray contact cleaner. biggest problem is access to them you have to get the spray inside some times the offending device is not accessible without de-soldering it. an intermittent open device is very hard to find. Good luck

My experience with amps usually has one of the audio channels dropping out, but never both. Since the thing looks like it can handle a lot of power - could there be a fuse in there (not so sure though)?

If you crank the volume to the top - do you get any output such as hiss or hum, or is there no signal at all coming out of the thing?

I too have an unhappy amp. It's been a while since I've looked at it, but I'm fairly sure only one of the channels is gone- what does that signify? I also seem to remember one of the channels sucking in the speaker cone when it was hooked up. I managed to get sound playing by manually switching a relay inside the amp, but the sound was poor. (I'm guessing the relay is switched off because the amp knows something is wrong.) It's an old sony TA-F3A if that helps.

I think you've asked the question incorrectly. The way to solve the problem is to get a highly skilled electronics technician to look at it when it stops working (and actually trace the circuit). As you have posed the question, you haven't provided enough detail. When you make unexplained observations, you have to provide as much detail as possible - even on the things that you don't think have an effect. You have to list how many times this has happened, how it stopped working - eg did it stop during playing (what signal?), or when turning on. What drivers are you using? Was the amplifiers protection mode tripped. Is your house hold power in spec? What vaccum did you use, which component did you vacuum, how much suction etc. If you list all of the variables, you know the answer has to be there somewhere. Of course to guess, it is most likely a bad circuit board connection somewhere. Amplifiers can have dry joints even if they look ok visually. But intermittent problems can also be due to aging capacitors, oxidizing coils and so forth. Sometimes on more sophisticated amps you have to leave it disconnected for a while to let the protection mode reset before it works again.

Could it maybe be that it's just like a NES - you just have to get _every_ dust particle out? Maybe you could test that by using canned air or a compressor to blow air instead of vacuum next time it happensl.

It could be a heat issue. Next time it quits working, shove it in the fridge without opening it. Let it cool down and see if it works then. This is similar to trebuchet03's suggestion of using freeze spray, though using the fridge (obviously) cools everything at once. Use freeze spray to isolate the bad chip if cooling in the fridge "fixes" it.

Next time it stops working.... Grab a can of air duster stuff. Open the cover and spray the air duster while holding the can upside down. Try and target individual components to get them nice and cold. I personally would do this while it's on and attempting to play music so if it starts working after hitting a single component, you've isolated the troublesome chip (if there is one).

If all channels don't output, it's probably not the amplifier chips (which are probably the ones on the big heat sinks).

Just curious - does it work in reverse? That is, can you blow air and get it to work again?

It doesn't really answer the question - but it seems the thing is failing more frequently now...

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Bio:Eric J. Wilhelm is the founder of Instructables. He has a Ph.D. from MIT in Mechanical Engineering. Eric believes in making technology accessible through understanding, and strives to inspire others ...read more »